You did the right thing to let him know how you felt and what you observed. He might have been honest but as his girlfriend, he shouldn't have treated you less than the other one.
You don't need to apologize. But, you can tell him that you are sorry if what you said has upset or hurt him. Nonetheless, let him know you have also been hurt by the fact he cared more about the other person though you didn't necessarily mean he was dishonest.
In any case, sing it is something you observed and his ending the relationship is a signal of the fact. You don't need to be discouraged. Just maintain no contact and try moving on.
Breaking up wasn't a wise thing. He should have approached your concern in a mature manner than ending the relationship.
I know you love him but him ending the relationship, throws things into array. Exercise no contact and at the end of it, you will have a good idea what to do next - move on or try to reunite.